Field of the Invention
The present invention relates to a thermal printing medium comprising a heat sensitive layer and a protective layer disposed on a substrate. More specifically, the present invention relates to a thermal printing medium exhibiting excellent printing stability such as no blurring of letters, high sensitivity, excellent water resistance, resistance to water dissolved plasticizer, as well as excellent chemical resistance and oil resistance. In particular, the present invention relates to a thermal printing medium having suitable characteristics for a thermally printed-type label, and a method for preparing the same.
In general, a thermal printing medium having a heat sensitive layer chiefly comprised of a colorless or a light colored leuco dye, and a color developing agent which imparts color to the leuco dye by thermally reacting with the leuco dye, is disclosed in Japanese Patent First Publication Serial No. 45- 14035. Such a thermal printing medium is used in a great variety of printing applications. In order to print on this thermal printing medium, a thermal printer device with a built-in thermal head is used. Such a thermal printing technique has many advantages, such as producing low noise, requiring no fixing development process, requiring little maintenance, is relatively inexpensive, may be of compact design, and the color of the produced images is very clear compared to that of other ordinary printing techniques. Therefore, thermal printing media are used in a great variety of printing applications, including computer hard copy, facsimile devices, numerous types of measuring instruments which produce printed output, and labels.
However, because the color-producing reaction in the heat sensitive layer, wherein a reaction occurs between the leuco dye and the color developing agent, is reversible, when the thermal printing medium is used under extreme conditions or when the medium contacts certain chemicals, the color-producing reaction may be readily reversed. As a result, the colored images may disappear. Therefore, it is difficult to maintain the thermal printing medium in good condition. In fact, for example, when the thermal printing medium contacts plasticizer included in wrapping films comprised of polyvinyl chloride, fats and oils included in edible lipids, industrial oils, or adhesive agents included in adhesive tapes or glue sticks, the colored images may readily disappear. In order to more widely use the thermal printing medium by improving the chemical resistance thereof, Japanese Patent Second Publication Serial No. 57-188392 for example, proposes a thermal printing medium in which a protective layer is formed over the heat sensitive layer to prevent the penetration of lipophilic chemicals such as plasticizers, oils or the like into the heat sensitive layer.
The stability of images of the thermal printing medium against the lipophilic chemicals of edible fats and the like is improved by forming the protective layer over the heat sensitive layer. However, in the case in which the thermal printing medium is used a label on food, when the label is soaked in water for a prolonged period, the water resistance of the label decreases. Moreover, because the plasticizer included in the food wrapping film diffuses into water and adheres to the label, resistance to water dissolved plasticizer of the food wrapping film is decreased. Therefore, the stability of preservation of the thermal printing medium is not improved satisfactorily by forming the protective layer over the heat sensitive layer. Because the substrate is made of paper and the protective layer is made of aqueous resin namely water soluble resin or water dispersed resin, the water resistance of the substrate and the protective layer is high. Therefore the heat sensitive layer is influenced easily with water or plasticizer dissolved in water.
Moreover a color developing agent is used to improve above-mentioned high water resistance. For examples, as the color developing agent, in particular 2,2'-bisphenolsulfone and 2,2'-bisphenolsulfide compounds are used (Japanese Patent Second Publication Serial No.56-30896). Certain kind of 4,4'-bisphenolsulfide compounds are used (Japanese Patent Second Publication Serial No.57-41996). Bis(3-allyl-4-hydroxyphenyl)sulfone is used (Japanese Patent Second Publication Serial No.60-208286). Many other examples suggest the compounds as the color developing agent. However, when above-mentioned compounds are used as the color developing agent, the water resistance is improved, but the fog appears in the background of the medium. Moreover, when the protective layer is formed over the heat sensitive layer, printing sensitivity and printing density of the thermal printing media are deteriorated.
In this way, in spite of many countermeasure are tried to make excellent thermal printing media, in fact the thermal printing medium having satisfactory printing sensitive characteristic, excellent chemical resistance and oil resistance, little fog of background, as well as excellent water resistance and resistance to water dissolved plasticizer, cannot be obtained.